Dramatic hijacking of a sentence

A common trope in movies. What's it called?

Person A: The President was a brilliant man! A truly one-of-a-kind--

Person B: killer, who used his ruthless abandon to get ahead!

Even TVTropes does not have a specific name for this, but rather sorts it under Finishing Each Other's Sentences:

Closeness and familiarity aren't the only reason for [Finishing Each Other's Sentences]. Others include:
[...]

• Last-Second Word Swap (especially when the audience was expecting it to rhyme):
  Alice: That sexy young farmer has an enormous—
  Bob: Potato gun!
• Making a quick gag by having the finished sentence be nothing at all like what the first person was going to say.
  Alice: Maybe we should—
  Bob: Tie a banana on its nose and conga under its legs!
  Alice: I was going to say call the police.

As you see, the latter is pretty much what you're looking for but does not have a dedicated name, and the former Last-Second Word Swap subcategory sounds enticing, but if you follow the link you'll learn it's not quite the thing you are after. It also links to Curse Cut Short and Subverted Rhyme Every Occasion, only getting you further away still.

So you are stuck with using the generic Finishing Each Other's Sentences. When even the dedicated site for common tropes in movies does not have a dedicated term for your common trope in movies, you're left with either betting substantial amounts of money on your being out of luck, or with going on and creating the article "Dramatical Hijacking of a Sentence" to see if it sticks.

I'm not an English speaker, but using Portuguese logic I believe it could be called "abrupt rupture".

When you start to explain something and your ideas are broken, it makes a rupture in the dialog, starting another one.